This invention relates in general to apparatus for measuring physical characteristics, such as the number of particles, of a flowing fluid stream and more particularly, to such apparatus which includes automatic calibration and rapid cleaning capabilities.
The counting, sizing and analysis of very small particles entrained in a flowing fluid stream is important in a number of different fields. Determining the efficiency of filters used with drinking water, evaluating contamination, such as by Giardia cysts, in a fluid stream, measuring biological particles in medical cytometers, etc., all require measurement of physical characteristics of the particles, such as the number and size of particles present.
Early methods included flowing the fluid between electrodes which have an imposed current such as is described by Coulter in U.S. Pat. No. 2,656,508 and by Groves et al in U.S. Pat. No. 4,298,836. Particles moving between the electrodes change the impedance of the sensing zone, which can be detected and correlated to particle characteristics. While this system and later variations are effective in many cases, typically in the analysis of biological cells, in general calibration is difficult, not all particle compositions can be effectively analyzed and the electrical system is complex.
Other apparatus which is intended for measuring particles labeled with fluorescent dyes use auto-fluorescent microbeads to calibrate the apparatus, as described in U.S. Pat. No. 5,093,234. The calibration method, however, is cumbersome and time consuming and limited in the types of particles that can be analyzed. Hogg et al in U.S. Pat. No. 4,286,876 describe a particle detection system in which particles, such as white blood cells, are passed through a sensing zone one-by-one. A beam of light is directed through the sensing zone, with photodetectors opposite the light source positioned to detect light scattered by each particle as it traverses the zone. With one light source and a single detector, identification of the particle is described as inefficient, complex and expensive. With a complex array of light sources and detectors, effectiveness improves but the system is still not capable of processing a stream of multiple particles.
Prior apparatus for analyzing particles entrained in fluid streams further have difficulty where there are large number of particles both in the analysis and in avoiding contamination of the analysis device, such as the window through which the stream is observed, to the point where clogging of the stream conduit may occur. Complex systems, such as that described by Simpson et al. in U.S. Pat. No. 4,220,621, have been designed for backwashing mixed fluids from a sampling probe. These systems are complex, not fully effective, and are generally not suitable for periodically cleaning an observation section of a conduit through which a continuous stream flows.
Thus, there is a continuing need for improved devices for measuring physical characteristics of multiple particles in a flowing fluid stream, typically continuously counting the number of particles in the stream. The need remains for a rapid, effective measurement method and apparatus which include the ability to rapidly and accurately calibrate the apparatus on a periodic schedule and which can be automatically cleaned on a regular schedule.